This application relates in general to the reading and writing of stored information and, in particular, to a system for reading and writing stored information through electrochemical changes.
With the advent of instruments such as the scanning tunneling microscope (STM), it is now possible to investigate the structure, spectra and dynamics of biological molecules and membranes as well as other substances at the atomic or molecular level. While more than a thousand STM's have been in operation and the instrument has sparked great interest in spectroscopy, the actual headway that has been made in this area remains rather modest. Thus, Bob Wilson and co-workers at IBM Almaden have made some progress in distinguishing closely related adsorbed surface species in STM images. G. Meijer et al., Nature 348, 621 (1990). In "Non-Linear Alternating-Current Tunneling Microscopy," Kochanski, Physical Review Letters, 62:19, pp. 2285-2288 (May 1989), a method for scanning tunneling microscopy is described, where a non-linear alternating current (AC) technique is used that allows stable control of a microscope tip above insulating surfaces where direct current (DC) tunneling is not possible.
The STM has a counter electrode on which the sample to be investigated is placed and another electrode in the shape of a microscope probe with a tip placed at a small distance away from the sample surface. A DC or a low frequency AC signal is then applied across the pair of electrodes. The probe tip is then moved across the sample surface in a scanning operation and the changes in the current or voltage across the electrodes are monitored to detect the characteristics of the sample.
The distance between the probe tip and the counter electrode/sample is controlled by a piezoelectric driver in one of two possible modes: a constant current mode and a constant height mode. The current or voltage detected between the pair of electrodes is used to derive a control signal for controlling the piezoelectric driver in the constant current mode to change the distance between the probe tip and the sample so as to maintain a constant current between the electrodes. The voltage that has been applied to the piezoelectric driver in order to keep the tunneling current constant indicates the height of the tip z(x,y) as a function of the position (x,y) of the probe tip over the sample surface. A record of such voltages therefore indicates the topographical image of the sample surface. The constant current mode can be used for surfaces which are not necessarily flat on an atomic scale. A disadvantage of the constant current mode is the time required for the electronic and piezoelectric components in the feedback loop for controlling the piezoelectric driver; this response time sets relatively low upper limits for the scan speed.
To increase the scan speed considerably, the feedback loop response is slowed or turned off completely so that the probe tip is rapidly scanned at a constant average distance to the counter electrode irrespective of the contours of the sample surface. The rapid variations in the tunneling current are recorded as a function of location (x,y) to yield the topographic information of the sample surface. This is known as the constant height mode referring to the fact that the probe tip is maintained at a constant average distance from the counter electrode.
The constant height mode is advantageous over the constant current mode since it has a faster scan rate not limited by the response time of the feedback loop. Consequently, slow dynamic processes on surfaces can be studied. On the other hand, it is more difficult to extract the topographic height information from the variations of the tunneling current. Furthermore, unless the sample is atomically flat, the tip might crash into a surface protrusion of the sample. For a more complete description of the two operating modes of the STM's, please see "Scanning Tunneling Microscopy I," by H.-J. Guntherodt R. Wiesendanger (Eds.), Springer-Verlag, pp. 5-6.
In the article referenced above, Kochanski proposes to investigate insulating films by applying an AC current between the electrodes at frequency .omega. and the current between the electrodes at 3.omega. is detected. The AC signal is generated using a 2 GHz resonant cavity so that the frequency or frequencies of the signal applied to the STM electrodes and detected must be fixed in the scanning operation performed by Kochanski.
STM's have been used for applications in data storage. In "Atomic Emission from a Gold Scanning-Tunneling-Microscope Tip," by Mamin et al., Phys. Rev. Lett., Vol. 65, No. 19, pp. 2418-2421, Nov. 5, 1990, and in "Gold Deposition from a Scanning Tunneling Microscope Tip," by Mamin et al., J. Vac. Sci. Technol., B9(2), pp. 1398-1402, March/April 1991, a gold STM tip is used as a miniature solid-state emission source for directly depositing nanometer-size gold structures. First, the deposition speed is slow (of the order of one millisecond per bit) and imposes a lower limit on the writing speed despite the much faster scanning speed of the STM. The deposition features vary in size (from about 5 to 20 nanometers) so that one would have to assume the maximum possible size. If safe spacing is maintained between adjacent features for which maximum feature size is assumed, the density at which bits can be written using such deposition method is much reduced. Data storage by deposition is generally a cumbersome method where gold tips are consumed in the process and must be replaced.
Another method of chemical modification of a surface is disclosed in "Nanometre-Scale Chemical Modification using a Scanning Tunneling Microscope," by Utsugi, Nature, Vol. 347, pp. 747-749, Oct. 25, 1990. In this article, a STM is used to etch the surface of a mixed-ionic conductor (Ag.sub.x Se), producing selected patterns of grooves about 10 nm wide. The positive voltage bias is applied to the STM tip. According to Utsugi, "owing to the high ionic conductivity of silver selenide, it seems probable that a positive bias to the tip provides a flux of mobile Ag.sup.+ ions from the surface toward the underlying matrix (Ag migration), resulting in segregation of the Ag and Se." The migration of the silver ions exposes selenium ions which are then removed from the surface by reaction with ambient hydrogen. The above-described chemical modification by Utsugi, however, is irreversible. For this reason, it cannot be used for rewritable data storage which requires the erasing of data that has been stored by a writing process. Furthermore, Utsugi's method appears to require the use of a mixed-ionic conductor which can be etched by the application of a DC voltage and reaction with an ambient or other gaseous atmosphere.
Data recording and erasing are disclosed in "Nanometre-Scale Recording and Erasing with a Scanning Tunneling Microscopy, "by Sato et al , Nature, Vol 363, pp. 431-432, Jun. 3, 1993. In view of the irreversible nature of the method by Utsugi, Sato et al. proposed the recording and erasing of data by means of phase transitions. A tunneling current is established between the STM tip and the surface of a composite medium when a bias voltage modulated by pulsed voltages are applied to the tip. The tunneling current established creates marks on the surface which were then erased by applying a reverse-polarity pulse. The authors Sato et al. "attribute the recording process to a phase transition from the amorphous to the crystalline phase." The method disclosed by Sato et al., however, can only be repeated about ten times. According to the authors, "any recording mark subjected to writing and erasing pulses more than 10 times became impossible to erase."
None of the above-described methods is entirely satisfactory. It is therefore desirable to provide an improved system for writing and erasing information in which the above-described difficulties are overcome.